The present invention relates to the field of image correlators.
The joint transform correlator (JTC) is well known to be one of the most convenient devices for correlating two images, since there is no need to fabricate separate holographic filters, such as the matched filter or the phase only filter. The classical joint transform correlator requires a quadratic processor in the Fourier plane. In the last decade many scientist have shown that it is possible to introduce nonlinearties in the image plane in order to improve the performance of these correlators. The joint transform correlator has been implemented so far in two ways. The first approach is based on using a camera as the nonlinear square low receiver, and a computer for digital image enhancement as well as for interfacing with other spatial light modulators, and finally a spatial light modulator to receive the processed data from the computer. The second approach is based on using an all optical spatial light modulator.
The disadvantages in the use of these approaches are that: most the spatial light modulators are binary in performance, which limit the use of many of the algorithms for image enhancement in the fourier plane; secondly, all the pixelated spatial light modulators have limited resolution; thirdly those based on using digital processing have serious limitations on speed due to the intermediate digital processing. To overcome these problems we employ a new category of nonlinear joint transform correlator which is based on using real-time holography. These correlators can be readily tuned from the matched filter to the phase-extraction limit and thus enhance signal detection in noisy environments by compressing the spectra of both the signal and the noise.
Our first design was based on energy transfer in a two-beam coupling JTC using barium titanate; see U.S. Pat. No. 5,493,444 to Khoury et al., and J. Khoury et al., Optical Society of America. B, 11, 11(1994). However, many fast real-time holographic materials, such as polymers and certain geometries of multiple quantum wells, cannot be used in this design because they do not produce two-beam coupling. Our second design was based on four-wave mixing as an alternative to two-beam coupling, utilizing a self-pumped phase-conjugator to retroreflect the joint spectra. See J. Khoury et al., Applied Optics, 33, 35 (1994). This design does not at present appear attractive because efficient self-pumped phase conjugation is slow. We therefore proposed a grating erasure JTC based on the incoherent erasure of real-time holograms.